In a consumption ultrasonic flow meter of the transit time type, the determination of the consumption is based on a flow rate measurement of the consumed medium. The flow rate measurement is based on measuring a transit time difference between counter-propagating ultrasonic waves in the medium flow. This time difference is normally performed by measuring the travel time of a wave packet travelling through a measurement distance of the medium. On a general level, the time measurement is performed by starting a clock when the wave packet is emitted from one transducer, and stopping the clock when the wave packet is received at the other transducer. The typical transit times of the ultrasonic signals through the medium are very short, and therefore the transit times, or at least the time differences between travel times of counter propagating wave packets, need to be measured with a very high accuracy. Due to this high accuracy, the specific implementation of the time measurements is critical for the system performance. As a consequence a number of time measurement implementations have been devised in the art.
In U.S. Pat. No. 6,226,598 B 1, the time measurement is based on digitizing the received signal, and performing digital signal processing on the received digitized signal to identify characteristic signal features in order to determine a specific signal feature from which an arrival time can be determined. The specific signal feature is in the form of a zero-crossing of the received wave package.
WO 97/14936 A1 disclose a similar methodology where the received signal is digitized and digital signal processing is used to extract a specific signal feature from which an arrival time can be determined. The specific signal feature is slopes of the waveform of the received wave package.
Signal transformation from the transmitter to the receiver can make it challenging to extract the parameters which are important to the flow rate calculation. The transformation is a result of filtering by the various components involved in the flow measurement. In particular, the electro-mechanical parameters of the piezo-electrical transducers normally lead to a dramatically change of the signal shape. Moreover, transducer characteristics can vary from unit to unit, and change over time as a consequence of aging, temperature variations, etc. resulting in that the shape and frequency content of the received signal can vary somewhat during the life span of the meter. Hence it would be advantageous to acquire a digital representation of the signal in order to utilize the tools available in Digital Signal Processing (DSP).
To obtain a detailed signal representation in order to make a precise time-of-arrival determination, a high-speed analogue-to-digital converter ADC may be used. High-speed ADCs are however expensive and consume a relative high amount of power, and therefore jeopardize the battery life time of battery powered meters, and becomes an important component for the price level of the meter.